1. Field of the Invention
The present invention relates generally to coffee makers including hot water feeding means such as a heating pipe provided between a water reservoir and a drip case for containing coffee powder and heating means for applying heat to the hot water feeding means to heat water therein, thereby feeding the hot water to the drip case by the force of vapor pressure at the boiling point (boiling pressure), for the extraction of coffee, and more particularly to coffee makers incorporating a mechanism for detecting and informing an operator of an abnormal state of the hot water feeding means with regard to the feeding of hot water.
2. Description of the Prior Art
The coffee makers of the above-mentioned type generally comprise hot water feeding means such as a heating pipe and heating means or a heater. One of two ends of the heating pipe communicates to the bottom of a water reservoir and the other end thereof is positioned over a drip case for containing coffee powder. The heater is attached to a suitable portion of the heating pipe so that water in the heating pipe is heated. When a user makes coffee with the above-described coffee maker, the heater is energized after the drip case is supplied with a desired amount of coffee powder with the water reservoir filled with water. Upon energization of the heater, the water supplied from the water reservoir to the heating pipe is heated. The boiling pressure causes the hot water to flow through the heating pipe to the end thereof positioned over the drip case. The hot water is dripped down into the drip case in which the coffee powder is contained. The above-described hot water feeding operation is reiterated and completed when all of the water in the water reservoir is heated and fed into the drip case. The hot water thus fed into the drip case is brought into contact with the coffee powder, thereby obtaining coffee extracts. The hot water containing the coffee extracts is dripped down into a container provided below the drip case, thereby obtaining coffee.
According to the above-described coffee maker, a small amount of water is left in the heating pipe even after the extraction of coffee. Since a remaining heat is applied to the water left in the heating pipe, the water left in the heating pipe is heated and boiled. However, when the coffee maker is reiteratively used for a long period, the reiterative remaining of the water in the heating pipe and the boiling of it result in the affixing of incrustations and the like on the inside surface of the heating pipe, thereby reducing an effective cross sectional area of the heating pipe. When the effective cross sectional area of the heating pipe is decreased, the amount of hot water flowing in the heating pipe is decreased, thereby prolonging the period of time required to extract coffee. Further progress of the affixing of the incrustations and the like results in further decrease of the water flowing in the heating pipe, thereby decreasing an amount of heat consumed. Consequently, the circumference of the heater is in danger of being exposed to excessive heat.
Whereas, the prior art has provided a coffee maker wherein a temperature-sensing switch is provided in the vicinity of the heating pipe so that the heater is automatically deenergized. In the case of the coffee maker of this type, when all of the water in the water reservoir is fed into the heating pipe and dripped into the drip case or when the drip operation is completed, the temperature around the heating pipe is rapidly increased. The temperature-sensing switch is automatically operated to deenergize the heater, in response to the rapid increase of the temperature around the heating pipe.
Should the effective cross sectional area of the heating pipe incorporated in the above-described coffee maker having the temperature-sensing switch be decreased owing to the incrustations and the like, the temperature around the heater would excessively be increased as mentioned above. Such excessive increase of the temperature around the heater causes the temperature-sensing switch to mistakenly operate. Consequently, the operation of the temperature sensing switch results in malfunction of the heater, that is, the heater is deenergized before all of the water in the water reservoir is fed or before the drip operation is completed.
Furthermore, when the affixing of the incrustations on the inside surface of the heating pipe progresses, impurities or foreign matter contained in the incrustations degrades the flavor of coffee to be made.